Researchers from LPS and LPEM (ESPCI) studied on the SWING beamline the self-assembly of semiconductor (CdSe and CdTe) nanoplatelets in solution. These ultra-thin anisotropic nanoparticles possess unique optical properties that could be exploited in optoelectronic devices such as LEDs or in the construction of photovoltaic cells.
When excited by ultraviolet light, those nanoparticles re-emit perfectly monochromatic light, the wavelength of which depends solely on their thickness. By varying the synthesis conditions in solution, it is possible to obtain plates whose thickness is controlled to the nearest atomic plane.
At the SWING beamline were studied by small angle X-ray scattering (SAXS) two types of platelets whose thickness differed only by a fraction of a nanometer. However, these platelets behave very differently. Thin plates have a lateral extension of about 100 nm and roll themselves up to form multi-walled tubes. Playing on their surface chemistry it is possible to unroll them and to obtain rectangular plates (ref1). These two structures have very different signatures with small angle scattering (see figure).
Thicker nanoplatelets (ref 2,3) are less laterally extended and tend to stack up on each other to form supercrystals exhibiting long-range lamellar order. This self-organization is visible in the scattering patterns through the presence of a series of very marked Bragg peaks, while the scattered intensity decreases monotonously when the platelets are dispersed. Furthermore, the scientists showed that this stacking had a significant impact on the optical properties of the nanoplatelets. Indeed, when the plates are stacked up, photons emitted by one are easily reabsorbed by a neighboring platelet. Thus, this self-organization makes a second peak visible in the emission spectrum.
These experiments allowed to study the structure of platelets in solution and provide an averaged measure over a large number of objects. In this respect, they are perfectly complementary to electron microscopy.
References :
1. Bouet, C. et al. Two-Dimensional Growth of CdSe Nanocrystals, from Nanoplatelets to Nanosheets. Chem. Mater. 25, 639–645 (2013).
2. Tessier, M. D. et al. Phonon line emission revealed by self-assembly of colloidal nanoplatelets. ACS Nano 7, 3332–3340 (2013).
3. Pedetti, S. et al. Optimized Synthesis of CdTe Nanoplatelets and Photoresponse of CdTe Nanoplatelets Films. Chem. Mater. 25, 2455–2462 (2013).